These type of drilling units are used e.g. when drilling holes when building of aircraft fuselage. The drilling units are mounted in frames or boring jigs, which are moved between the different boring sites. Depending on the type of material that is processed, and also material thickness, there is a need for varying the automatic drill shaft feeding velocity. According to prior art, the drilling units have to be rebuilt in order to obtain different feeding velocities. One reason for this is that the drilling units have to be kept small because of the mostly narrow working space around an aircraft fuselage. Thus, a gearbox built into the unit would make it too large. There is a need for drilling units with variable automatic drill shaft feeding velocities.